In ranked-choice elections, voters vote by indicating their preference orderings over the candidates. A ballot is truncated when the ordering is incomplete (called partial voting). Sometimes truncation is forced—voters are allowed to rank only a limited number of candidates—but sometimes it is voluntary. During the vote tabulating process, a truncated ballot is exhausted when all of the candidates it ranks have been eliminated. Ballot exhaustion and, therefore ballot truncation, is a concern in single-winner elections when the margin of victory in the final stage is less than the number of exhausted ballots. That concern motivates our study. We review evidence from actual single-winner ranked-choice elections and conclude that voluntary ballot truncation is very common. Moreover, it is difficult to explain strategically. To assess the significance of ballot truncation, we simulate ranked-choice elections with four, five and six candidates, using both spatial and random models of voter preferences. Does truncation change the probability that a Condorcet winner wins the election? Does the winner change as the extent of truncation increases? We find that even small amounts of truncation can alter election outcomes.
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Work supported by the Canadian Tri-Council Research Support Fund. The authors were supported by their own individual Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grants (Grant Nos. 5023, 155957-2012, 2015-06126).
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A.M. Foley: formerly Hamel.
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Kilgour, D.M., Grégoire, JC. & Foley, A.M. The prevalence and consequences of ballot truncation in ranked-choice elections. Public Choice 184, 197–218 (2020). https://doi.org/10.1007/s11127-019-00723-2